As is well known to those skilled in the design of color television receivers, the VIR signal, transmitted during the 19th line of each vertical frame, provides a useful means of correcting, both at the transmitter and within the receiver itself, errors in both the saturation and tint of the demodulated color signal. Although an elaboration on the exact composition of the VIR signal has been presented elsewhere (See, inter alia, U.S. Pat. No. 3,950,780 entitled "VIR Chroma System" to Harry T. Freestone) and is not necessary for an understanding of the subject invention, it suffices to note that the VIR signal includes separate chrominance reference, luminance reference and black level reference components. In addition, the chrominance reference component is characterized by the same phase angle as the color burst signal so that, given a properly phased 3.58 MHz subcarrier oscillator in the receiver, the R-Y and B-Y components of the chrominance reference signal will have quadrature (90.degree.) and a 180.degree. relationships to the reference oscillator, respectively. With this in mind, it can be seen that tint correction can be achieved by sampling the (R-Y) output of the chrominance demodulator during the chrominance reference interval.
The (R-Y) output during the chrominance reference interval is sampled and compared to a ZERO-COLOR reference signal, that is, the (R-Y) output during any time in which there is no chrominance information in the VIR signal, for example, during the black level reference interval. (The Freestone patent cited above describes such a system.) The voltage at the R-Y of the demodulator during the black level reference interval (or during the reception of any other ZERO-COLOR reference signal) will correspond to its quiescent level and should be equal, to the extent that the subcarrier oscillator is properly phased, to the voltage at that output during the chrominance reference interval. Consequently, these outputs, occuring at two different times, can be sampled and stored and used as inputs to a feedback loop that includes the subcarrier oscillator. In short, a difference in the two voltages results in an error correction voltage that controls the phase of the subcarrier oscillator in a manner that tends to reduce the difference to zero, thereby assuring proper phasing of the oscillator.
Although the system as described above is effective, its attractiveness is mitigated by the need for two time gates, one during the VIR chrominance reference interval and one during, for example, the black level reference interval. Other systems attempting to circumvent this requirement have used a fixed "ZERO-COLOR" reference signal that approximates the (R-Y) output of the demodulator during times when no chrominance information is present. However, although this system requires only one time gate, the desired "ZERO-COLOR" reference level depends on, and therefore must be adjusted to, the demodulator. Further, the reference level can be expected to be temperature dependent, with the dependence differing among particular demodulators.
Accordingly, what is desired and attained in this invention is a VIR Tint Control circuit affording not only reduced cost and complexity but also performance features associated with optional Tint Control.